RF linear amplifiers utilize devices that exhibit non-linear characteristics at higher power levels whereby signal distortion is introduced. If more than one signal is applied to an RF amplifier, its non-linear characteristics cause an unwanted multiplicative interaction of the signals being amplified and the amplifier output contains intermodulation products. These intermodulation products cause interference and crosstalk over the amplifier frequency operating range which interference may exceed established transmission standards.
As is well known, intermodulation distortion can be reduced by predistorting the signal to be amplified to cancel the amplifier generated distortion. Some predistortion techniques such as described in U.S. Pat. No. 4,465,980 issued to H. Huang et al Aug. 14, 1984 use non-linear elements to compensate for the distortion generated in the amplifier. The characteristics of the non-linear elements, however, generally differ from those of the amplifier to be corrected so that compensation is only effective over a limited range. Other arrangements such as described in U.S. Pat. No. 4,291,277 issued to R. C. Davis, Sept. 22, 1981, or U.S. Pat. No. 4,554,514 issued to J. C. Whartenby, Nov. 19, 1985, utilizes the base band converted modulation component of one input signal to produce predistortion but is not effective for systems having a plurality of concurrent signals or over an extensive range of frequencies.
U.S. Pat. No. 3,755,754 issued to J. L. Putz, Aug. 28, 1973, describes a predistortion compensation arrangement for a microwave amplifier in which a multifrequency signal is applied to a power amplifier through a variable phase shifter and through a distortion producing amplifier having substantially similar distortion transfer characteristics. The output of the distortion producing amplifier is combined with the undistorted signal from the variable phase shifter to form a predistortion signal having distortion components opposite in phase to the distortion components of the power amplifier whereby compensation is obtained. In similar fashion, U.S. Pat. No. 4,068,186 issued to G. Sato et al, Jan. 10, 1978, discloses a circuit for compensating for non-linear characteristics in high frequency amplifiers in which an input signal is divided into two parts. One part is applied to a first transmission line having a delay line and the other part is applied to a second transmission line having a distortion producing high frequency amplifier. The vector sum of the transmission line outputs provides a non-linear characteristic that compensates for amplitude and phase non-linear characteristics of a high frequency amplifier connected in cascade with the compensating circuit. While the use of a distortion generating amplifier with similar distortion characteristics to the amplifier to be corrected provides a significant improvement, the control of the predistortion circuits is preset so that variations in circuit element characteristics due to changes in operating conditions may adversely affect distortion compensation.
U.S. Pat. No. 4,453,133 issued to G. W. Travis, June 5, 1984, and assigned to the same assignee discloses an active predistorter for linearity compensation which also utilizes distortion characteristics of an amplifier similar to the power amplifier to be corrected. An input signal is divided into a part that is applied to a signal path having a linear delay, a part that is applied to the distortion producing amplifier and a part that is combined with a portion of the output of the distortion producing amplifier to generate a phase inverted predistortion signal. The phase inverted predistortion signal is combined with the linear delay path signal and applied to the input of the amplifier to be corrected. Optionally, a portion of the power amplifier output signal is fedback to provide a closer match in the operating characteristics of the amplifier to be corrected and the distortion producing amplifier. This arrangement, however, results in attenuation of the input signal through the predistortion circuit and does not provide continuous control of the attenuation and phase characteristics needed for distortion compensation. It is an object of the invention to provide an improved predistortion circuit having automatic control of distortion compensation of a wide band linear power amplifier without loss.